Ferroelectricity is the property of certain materials (also referred to as ferroelectric materials) that have a spontaneous electric polarization controllable by the application of an external electric field. For example, the direction of the polarization of a ferroelectric material can be changed by the direction of the external field. In other words, applying an electric field along one direction can align the polarization direction of the ferroelectric material along that direction, and applying another electric field in the opposite direction can reverse the polarization direction of the ferroelectric material.
Ferroelectricity can be used in constructing memory devices, in which the polarization state of the ferroelectric material can represent information. For instance, one polarization may represent a “0” bit and another polarization state can represent a “1” bit. Conventional memory devices using a ferroelectric material usually use out-of-plane polarization states in which the polarization is perpendicular to the surface of the ferroelectric material. In these devices, writing voltages are applied across the thickness of the ferroelectric material (i.e., perpendicular to the surface of the ferroelectric material) to change the polarization. The reading voltage is also perpendicular to the surface of the ferroelectric material to measure the polarization state of the ferroelectric material. The information written into the memory can be read out by detecting the polarization state of the ferroelectric material. This can be realized by applying a reading voltage that is also perpendicular to the surface of the ferroelectric material and then measuring the pulse current across the ferroelectric material in response to the reading voltage to determine the polarization state.
Conventional memory devices using out-of-plane polarization of ferroelectric materials have several drawbacks. First, both the writing voltage and the reading voltage are applied across the thickness of the ferroelectric material. Therefore, the reading voltage (and the reading process) can change the polarization state of the ferroelectric material. Accordingly, the reading process in these devices is destructive and a reset electric field is usually applied to maintain the initial polarization. This can increase the complexity and power consumption of the memory devices. Second, since out-of-plane polarization is used, the ferroelectric material usually has a thickness greater than a threshold thickness (e.g., at least several unit cells of the ferroelectric material) in order to build up the polarization field. This limits the smallest possible size of a memory device that uses out-of-plane polarization.